A mobile communication network standardized by the 3rd Generation Partnership Project (3 GPP) (hereinafter, referred to as 3G network) comprises, as shown in FIG. 1, CN (Core Network) 10 and UTRAN (Universal Terrestrial Radio Access Network) 11. UTRAN 11 has a plurality of RNS (Radio Network Subsystem) 12, 13. RNS 12, 13 consist of RNC (Radio Network Controller; wireless base station controller) 14, 15 and NodeB (wireless base station apparatus) 16, 17, 18, 19, respectively.
A user equipment (UE) (not shown), which is a mobile communication terminal, can switch the NodeB, which are connected by a wireless link, and continue the communication while moving. This is called as “handover.” The handover comprises a method (soft handover) in which downward data are copied in the RNC, the copied data are temporarily transmitted to a Source NodeB (handover originating base station) and a Target NodeB (handover destination base station) and the UE receives the data from both NodeB at the same time.
In addition, WO2004/030396 or Japanese Unexamined Patent Publication No. 2003-078937 discloses a technology in which downward data are buffered in the RNC, the data, which has not completely received by the UE from the Source NodeB, are again transmitted to the Target NodeB from the RNC and then transmitted to the UE from the Target NodeB to prevent omission of the downward data during the handover.
Meantime, for the purpose of improvement of a throughput of user data, reduction of delay of a call connection and transmission delay of user data, reduction of the number of nodes, reduction of an interface requiring the standardization and the like, the 3 GPP investigates a next generation (B3G: Beyond 3G) network (hereinafter, referred to as LTE/SAE network) that is called as LTE (Long Term Evolution) and SAE (System Architecture Evolution). As shown in FIG. 2, LTE/SAE network 20 comprises EUTRAN (Evolved UTRAN) 22 and Evolved CN 21. Nodes 23, 24 referred to as eNodeB are arranged in EUTRAN 22.
In LTE/SAE network 20, Evolved CN 21 and eNodeB 23, 24 are connected to each other by an interface called as S1 interface. In addition, eNodeB 23 and eNodeB 24 are connected to each other by an interface called as X2 interface.
In LTE/SAE network 20, it is investigated that when UE 30 performs the handover between the other eNodeB, data, which has not been completely transmitted to the UE, of the downward data transmitted to the Source eNodeB from Evolved CN 20, is transmitted to the Target eNodeB from the Source eNodeB and then to the UE from the Target eNodeB, thereby reducing occurrence of the data not to be transmitted.
Here, just after the handover, there exist in the Target eNodeB, as the downward data to be transmitted to the UE, the data transmitted from the Source eNodeB and the data directly transmitted to the Target eNodeB from the Evolved CN. Typically, when the TCP (Transmission Control Protocol) and the like are used in the upper layer, the change of the data sequence has influence on deterioration of a throughput. Thus, it is preferred to transmit the data to the UE in regular order as much as possible.
Additionally, since the data transmitted from the Source eNodeB is the data that has been transmitted toward the UE from the Evolved CN before the handover, it is preferred to transmit it to the UE before transmitting the data transmitted to the Target eNodeB from the Evolved CN, after the handover. However, the Target eNodeB does not always receive the data transmitted from the Source eNodeB before transmitting the data transmitted from the Evolved CN. Furthermore, the data sequence may be changed during the transmission.
Due to this, when the Target eNodeB desires to transmit the data to the UE in an order that the Evolved CN intends to transmit it to the UE, it is necessary that the Target eNodeB should rearrange, in regular order, the data transmitted from the Source eNodeB and the data transmitted from the Evolved CN and then transmit them.
As shown in FIG. 3, UE 30 may discontinuously complete to receive the downward data 1˜6 that has been transmitted to UE 30 from the Source eNodeB. In this case, although, as data 2 of FIG. 3, for example, a transmission confirmation (ACK: Acknowledge) was made in Source eNodeB 23, when the data (data 1), which should be transmitted before transmitting the corresponding data, is not completely transmitted, it is considered a method of transmitting data 2 to Target eNodeB 24 and a method of not transmitting it.
For a case that data 2 is transmitted to Target eNodeB 24, even though it has been completely transmitted to UE 30 from Source eNodeB 23, it is transmitted to Target eNodeB 24, so that the network between both eNodeB is consumed. Further, the data may be again transmitted to UE 30 from Target eNodeB 24, so that waste occurs.
In addition, for a case that data 2 is not transmitted to Target eNodeB 24, Target eNodeB 24 discontinuously receives the data transmitted from Source eNodeB 23 (i.e., it receives data 3 after receiving data 1). Thus, Target eNodeB 24 cannot determine whether Source eNodeB 24 discontinuously transmits the data or there occurs loss or delay on the network with respect to data 2. Regarding this, it is required a process of waiting for a receiving by starting a timer that is pre-provided.
Meanwhile, a Japanese Unexamined Patent Publication No. 2003-078937 discloses that when a base station is switched due to handover, a buffer state synchronizing signal is transmitted and received between base stations so as to prevent the data from being duplicated or lost. However, in order to transmit and receive the buffer state synchronizing signal in addition to the data transfer, the throughput is increased and the communication network should be consumed between both base stations.